Abstract
Apatite and biotite from syenitic intrusions at the Karari gold deposit, Kurnalpi Terrane, Archean Yilgarn Craton, Western
Australia, were targeted for microprobe and LA-ICP-MS analyses in order to determine some parameters (e.g., temperature,
oxygen fugacity) during magmatic crystallization and potassic (biotite) hydrothermal alteration. The understanding of
magmatic conditions is limited by the almost complete hydrothermal modification of magmatic minerals but made possible by the analysis of refractory magmatic apatite and rare relics of Ba-rich biotite. The SO3 contents of apatite cores (up to 1 wt.%) indicate that the syenitic magmas were strongly oxidized (fO2 . NNO þ1) and contained approximately 1360 ppm S. The Cl
contents of apatite are very low, suggesting crystallization from a low-Cl magma resulting from early separation of a volatile
phase that sequestered Cl from the magma. Trends defined by wt.% SO3, at constant XF/XCl, are consistent with partitioning of
oxidized S into the volatile phase during apatite crystallization.
Early potassic alteration of the syenitic intrusions produced fine-grained seams of hydrothermal biotite and compositional
modifications of igneous biotite phenocrysts. The magnesian composition of the hydrothermal biotite (mg# . 0.5) suggests that
oxidized magmatic conditions persisted during potassic alteration. Provinciality of compositional data for different intrusion
samples suggests that potassic alteration took place at low fluid:rock ratios. The potassic hydrothermal fluid modified the rims
of igneous apatite crystals, depleting them in F, light rare earth elements, Mn, and S. The rare earth elements were rapidly
redeposited, as monazite, in the apatite rims or surrounding groundmass. The depletion of S in apatite rims indicates that the
potassic (biotite) alteration fluid was under-saturated in S. Application of the biotite-apatite geothermometer suggests potassic
(biotite) alteration took place at temperatures of about 660 8C. The results of this study suggest the syenitic magmas at Karari
were strongly oxidized, similar to Canadian examples of syenite-associated gold deposits, and therefore potentially fertile for
hydrothermal gold.
Australia, were targeted for microprobe and LA-ICP-MS analyses in order to determine some parameters (e.g., temperature,
oxygen fugacity) during magmatic crystallization and potassic (biotite) hydrothermal alteration. The understanding of
magmatic conditions is limited by the almost complete hydrothermal modification of magmatic minerals but made possible by the analysis of refractory magmatic apatite and rare relics of Ba-rich biotite. The SO3 contents of apatite cores (up to 1 wt.%) indicate that the syenitic magmas were strongly oxidized (fO2 . NNO þ1) and contained approximately 1360 ppm S. The Cl
contents of apatite are very low, suggesting crystallization from a low-Cl magma resulting from early separation of a volatile
phase that sequestered Cl from the magma. Trends defined by wt.% SO3, at constant XF/XCl, are consistent with partitioning of
oxidized S into the volatile phase during apatite crystallization.
Early potassic alteration of the syenitic intrusions produced fine-grained seams of hydrothermal biotite and compositional
modifications of igneous biotite phenocrysts. The magnesian composition of the hydrothermal biotite (mg# . 0.5) suggests that
oxidized magmatic conditions persisted during potassic alteration. Provinciality of compositional data for different intrusion
samples suggests that potassic alteration took place at low fluid:rock ratios. The potassic hydrothermal fluid modified the rims
of igneous apatite crystals, depleting them in F, light rare earth elements, Mn, and S. The rare earth elements were rapidly
redeposited, as monazite, in the apatite rims or surrounding groundmass. The depletion of S in apatite rims indicates that the
potassic (biotite) alteration fluid was under-saturated in S. Application of the biotite-apatite geothermometer suggests potassic
(biotite) alteration took place at temperatures of about 660 8C. The results of this study suggest the syenitic magmas at Karari
were strongly oxidized, similar to Canadian examples of syenite-associated gold deposits, and therefore potentially fertile for
hydrothermal gold.
| Original language | English |
|---|---|
| Pages (from-to) | 217-238 |
| Number of pages | 22 |
| Journal | Canadian Journal of Mineralogy and Petrology |
| Volume | 61 |
| Issue number | 2 |
| DOIs | |
| Publication status | Published - 2023 |